The present invention relates generally to electronic systems and more particularly is directed to memory modules.
Memory modules are well known in the art and are commonly used in a wide variety of electronic systems, such as computer systems, to store data.
Memory modules typically comprise a circuit board, such as a printed circuit board, and one or more electronic components, such as integrated circuit (IC) memory chips, which are mounted directly on the circuit board. The circuit board is a substrate which electrically connects the electronic components mounted thereon in a desired configuration to form the functional memory module.
As noted above, memory modules are commonly used to store data for computer systems. Accordingly, memory modules typically include a plurality of connection points, such as a comb of printed terminals, formed along one edge which serve to electrically connect the memory module to the motherboard of the desired computer system. Specifically, memory modules are typically constructed in such a manner so that the plurality of connection points can be electrically and mechanically coupled to the motherboard of the desired computer system through a corresponding electrical connector.
It should be noted that the number and configuration of the connection points, or pins, on the memory module circuit board are commonly used to classify the memory module (e.g., a single in-line memory module (SIMM), dual in-line memory module (DIMM), etc.). Presently, computer industry standards mandate that computer systems be able to receive memory modules having a particular configuration and number of connection points (e.g., a 184-pin dual in-line memory module).
Designers of memory modules are constantly striving to increase, or expand, the memory capacity, or density, of memory modules. However, as noted briefly above, industry promulgated standards mandate that memory modules be designed to mechanically and electrically engage with a particular type of computer system electrical connector. Furthermore, because computer system designers typically limit the space within the housing of a computer system that is designated to receive a memory module, memory module designers are typically required to expand the module density of memory modules without significantly increasing its overall size. As a result, memory module designers are required to increase the memory capacity of memory modules while enabling each module to fit within the footprint of an industry-promulgated electrical connector and, at the same time, within the relatively confined space within the interior of the computer system which is designated for the memory module.
Design engineers utilize numerous well-known techniques to increase the memory density of a memory module without changing the size and/or connection point configuration of its circuit board.
As an example, memory module designers often use circuit boards which allow for electronic components to be mounted on both of its sides (this type of circuit board being referred to simply as a double-sided circuit board in the art). As can be appreciated, double-sided circuit boards enable design engineers to increase the number of electrical components which can be mounted on the circuit board, thereby increasing its overall density, without changing the size of the circuit board, which is highly desirable.
As another example, memory module designers often utilize individual electronic components of reduced size and of increased memory capacity, which is highly desirable.
As another example, memory module designers have, on occasion, coupled together multiple memory modules through the use of rigid flex technology. Specifically, a primary circuit board comprising the plurality of connection points is electrically coupled to a secondary circuit board through a flex circuit, each of the primary and secondary circuit boards having a plurality of electronic components mounted thereon. It should be noted that the flexibility of the flex circuit enables the primary and secondary circuit boards to be closely disposed in a substantially parallel configuration.
In U.S. Pat. No. 5,224,023 to G. W. Smith et al, there is disclosed an electronic assembly which combines a number of commensurate printed circuit boards that are bonded to a common, flexible, interconnecting substrate in an alternately folded and layered arrangement against an end board that has a comb of terminals for mounting into a motherboard connector. The flexible substrate is sandwiched between half-sections of each board, allowing mounting of components from both faces of the board. The assembly is particularly indicated for high density applications such as memory modules.
Although effective in increasing module density while maintaining the same height and electrical connector footprint as a memory module comprising a single printed circuit board, memory modules of the type described above which use rigid flex technology to connect multiple circuit boards together experience a few notable drawbacks.
As one drawback, memory modules which utilize rigid flex technology have been found to have a limited operating speed, which is highly undesirable. Specifically, it has been found that the speed in which signals can travel through the flex circuit is significantly limited. As such, a signal sent from an electronic component on the secondary circuit board reaches the comb of printed terminals on the primary circuit board significantly later (e.g., approximately 2 nanoseconds) than a signal sent from an electronic component on the primary circuit board. As a result, the memory module has a speed threshold which is limited to the speed in which signals can travel from the electronic components on the secondary circuit board to the comb of terminals on the primary circuit board. As can be appreciated, the calculated speed threshold for memory modules which utilize rigid flex technology has been found to be unacceptable in many present applications.
As another drawback, memory modules which utilize rigid flex technology have been found, at times, to perform inadequately, which is highly undesirable. Specifically, due to the significant length of the flexible substrate which connects the primary and secondary circuit boards, a signal generated from an electronic component on the secondary circuit board is required to travel along a relatively long trace length which, in turn, can result in integrity loss to the signal. As can be appreciated, it has been found that signals which travel along a trace length greater than 1.5 inches often experience significant integrity loss.
As another drawback, memory modules which utilize rigid flex technology have been found to be relatively expensive to manufacture, which is highly undesirable. Specifically, the cost associated with the flexible substrate is considerably high and, as a consequence, greatly increases the total overall cost of the memory module.
As another drawback, memory modules which utilize rigid flex technology have a considerably greater width (i.e., thickness) than other types of conventional memory modules, which is highly undesirable.
Accordingly, in order to increase the memory density of a memory module without changing the number, type and/or size of its circuit boards, memory module designers often assemble stacks of multiple (e.g., two, four, etc.) electronic components and, in turn, mount the stacks directly onto the circuit board. When utilizing this technique, design engineers commonly stack electronic components (i.e., integrated circuits) having a thin small outline package (TSOP) with a dual in-line lead configuration. It should be noted that when using this technique, each electronic component is individually manufactured, each component comprising a memory die which is at least partially encapsulated within a thin small outline package which includes a plurality of externally-accessible conductive pins. During the process of manufacturing of the memory module, the individual electronic components are stacked one on top of another such that the pins of every component in a stack are electrically connected together. In other words, the pins for each successively stacked component are connected to the pins of the component on which it is stacked (either directly or through a thin printed circuit board). The stacked components are then connected to the printed circuit board for the memory module.
As can be appreciated, by stacking TSOP electronic components on top of one another, design engineers are able to significantly increase the memory capacity of memory modules without increasing the overall height or length of the module and without compromising the signal speed capabilities of the memory module, which is highly desirable.
However, memory modules of the type described above which include stacked TSOP electronic components suffer from a few notable drawbacks.
As one drawback, memory modules of the type described above which include stacked TSOP electronic components often experience unacceptable signal degradation, which is highly undesirable. Specifically, stacking multiple TSOPs on top of one another creates a significantly long trace length from the TSOP at the top of the stack to the circuit board. As can be appreciated, the increased trace length serves to add inductance and capacitance to the stack which, in turn, unacceptably delays a signal traveling from the stack to the circuit board.
As another drawback, memory modules of the type described above which include stacked TSOP electronic components are considerably greater in width (i.e., thickness), which is highly undesirable. Specifically, stacking electronic components on both sides of a circuit board serves to increase the overall width of the memory module by roughly the increase height of the largest stack on each side of the circuit board. As noted above, due to space constraints, stacking electronic components on both sides of a circuit board often precludes such a memory module from fitting within the space within a computer which is designated for the memory module.
Another technique which is commonly used to increase the memory density of a memory module involves maximizing the number of electronic components which can be fit onto the front and rear surfaces of a standard memory module circuit board.
One well-known method for maximizing the number of electronic components which can be fit onto a circuit board involves the use electronic components which include a ball grid array (BGA) package. Specifically, a BGA package utilizes a plurality of conductive balls mounted on its bottom surface for electrically connecting the die within the package of the electronic component to the circuit board.
The utilization of electronic components which include a BGA package introduces a couple of notable advantages.
As a first advantage, the solder balls which are used to connect the die of the electronic component to the circuit board are connected to the bottom surface of the electronic component package. To the contrary, a dual in-line package utilizes a plurality of leads mounted along the sides of its package for electrically connecting the die of the electronic component to the circuit board. As can be appreciated, by mounting the conductive balls onto the bottom surface of its package, rather than along the sides of its package, the footprint required on the circuit board for each electronic component is significantly reduced. As a result, the number of electronic components which are capable of being mounted onto the circuit board is maximized.
As a second advantage, because the surface area of the bottom surface of the package is traditionally larger than the surface area of the sides of the package, by mounting the solder balls to the bottom surface, the electronic component is able to significantly increase the number of points of electrical contact between the die of the electrical component and the circuit board, which is highly desirable.
Another well-known method for maximizing the number of electronic components which can be fit onto a circuit board involves mounting each electronic component onto the circuit board without any packaging. Specifically, the die of each electronic component is mounted directly onto the circuit board without any packaging. As can be appreciated, with the packaging removed, the footprint of each electronic component is significantly reduced in some cases, thereby enabling more electronic components to be fit on the circuit board, which is highly desirable.
The aforementioned technique of mounting the die of each electronic component directly onto the circuit board without any packaging introduces one notable drawback. Specifically, the lack of packaging for each electronic component renders it incapable of most types of pre-testing (e.g., speed testing, temperature testing, etc.). As a result, if only one package-free electronic component which is mounted on the circuit board is found to be defective, the entire memory module can be significantly compromised. This compromising often necessitates the disposal of the memory module, thereby wasting all of the other properly functioning electronic components mounted on the circuit board. The discarding of a large number of properly functioning electronic components can produce a cost associated yield loss which is unacceptable.
In all of the aforementioned techniques for increasing the memory capacity of a memory module, it is to be understood that each electronic component used comprises only a single memory die which is either encapsulated in within a package or mounted directly onto the circuit board without any packaging.
In certain applications outside of the memory module industry, such as the cell phone and hand-held computer industries, it is known for a pair of different dies, each die uniquely designed to perform a particular function (e.g., a first die for controlling a device, such as a keyboard, and a second die for storing data), to be stacked on top of one another and encapsulated within a single thin small outline package. By encapsulating the pair of application specific dies within a common integrated circuit package, the internal architecture of the system in which said component is used can be considerably simplified.